1. Field of the Invention
The present invention relates to apparatus for measuring the flatness of a surface by scanning the surface with a scanning beam and detecting the angular deviation of the beam of light regularly reflected from the surface.
2. Description of the Prior Art
Hitherto, various types of measuring apparatus have been used to measure the flatness of a surface. Among them, interferometer using the phenomenon of interference of light and electrostatic capacity type of measuring apparatus making use of change of electrostatic capacity are most widely known in the art.
While the interferometer is able to give two-dimensional information regarding the flatness, it has such particular disadvantages that an extremely high accuracy is required for setting the surface to be measured and that measurement of unevenness of the surface and quantative measurement of the surface are very difficult to carry out. On the other hand, the capacity type of measuring apparatus has such disadvantages that adjustment of capacity is required every time the kind of material of the test subject is changed from one to another although the apparatus has a merit of giving information of thickness of the subject.
In order to eliminate the above described disadvantages, there has been proposed another type of measuring apparatus in which a surface to be measured is scanned by a scanning beam incident upon the surface at an incident angle essentially normal to the surface and the angular deviation of the beam of light regularly reflected from the surface is detected to know the flatness of the surface. The inclination of the surface in the scanning direction, that is, the value of primary differentiation is detected as a positional deviation on a position detector photoelectric element. A computation of the differential value by an electrical integration gives the desired flatness of the surface. A typical example of such type of flatness measuring apparatus is disclosed in U.S. Pat. No. 3,885,875 specification.
This type of measuring apparatus has, however, a particular disadvantage. When the surface to be measured is a surface containing pattern area as in the case of a wafer having thereon patterns and when the cross sectional area (or beam diameter) of the scanning beam remains unchanged for different pattern areas, the quantity of light reflected from the subject surface into the collimator lens becomes insufficient to carry out the measurement due to the loss of light by diffraction and/or scatter of the reflected light. This problem is effectively solved by the present invention.